The present invention relates to a hydrodynamic type porous oil-impregnated bearing being impregnated with lubricating oil or lubricating grease in a bearing body of porous substance, such as sintered metal, to have a self-lubricating function, supporting a slide surface of a shaft in a non-contact manner by a lubricating oil film produced in a bearing clearance due to hydrodynamic function of hydrodynamic pressure generating grooves in a bearing surface. The bearing of the invention is suitable for use particularly in machines and instruments of which high rotation accuracy at high speed is required, such as spindle motors for polygon mirror of laser beam printer (LBP), magnetic disk drives (HDDs), or the like, and in machines and instruments which are driven at high speed with a large imbalance load produced in that a disk is mounted thereon, such as spindle motors for DVD-ROM, or the like.
In such small-sized spindle motors associated with information-handling devices, improved rotation performance and cost reduction are required, as a means therefor, possibility of changing bearings for the spindle from a rolling bearing to a porous oil-impregnated bearing has been investigated. However, since a porous oil-impregnated bearing is a kind of cylindrical bearing, it tends to produce unstable vibrations where the shaft eccentricity is small, inducing the so-called whirl in which the shaft is subjected to a revolving vibration at a rate which is half the rotary speed. Accordingly, it has heretofore been attempted to form hydrodynamic pressure generating grooves, such as the herringbone or spiral shape, in a bearing surface, so as to produce a lubricating oil film in a bearing clearance by the function of the hydrodynamic pressure generating grooves which accompanies the rotation of the shaft, to thereby support the shaft in a noncontact manner (hydrodynamic type porous oil-impregnated bearing).
A porous oil-impregnated bearing being formed hydrodynamic pressure generating grooves in a bearing surface is disclosed in Japanese Utility Model Koukoku Shouwa 63-19627. In this prior art, a region of the hydrodynamic pressure generating grooves in the bearing surface is worked to seal surface openings thereon. Such construction, however, has the following drawback. Since the surface openings on the region of the hydrodynamic pressure generating grooves completely sealed, the circulation of oil, which is the greatest feature of the porous oil-impregnated bearing, is obstructed. Therefore, the oil which has been exuded in the bearing clearance is forced into the bent portions of the groove region by the action of the hydrodynamic pressure generating grooves and stays there. A great shearing action is present in the bearing clearance, and this shearing force and frictional heat cause the oil staying in the groove region to be denatured, while a rise in temperature tends to accelerate oxidative deterioration of the oil. Therefore, the bearing life is shortened. On the other hand, besides plastic processing, it has been proposed to employ coating or the like as another means for applying a surface treatment, however, it is necessary that the thickness of such coating film be less than the groove depth, and it is very difficult to apply a coating film which is some xcexcm thick solely to the groove region.
In order to secure the rotation accuracy of the shaft, a plurality of bearings, e.g., two bearings, are usually used. Further, bearings are used mostly by being pressed into a housing. Thus, to secure a substantial alignment of the two bearings, there has been employed a method in which two bearings are simultaneously pressed into the housing after a correcting pin is inserted into the housing. In the case of a bearing having hydrodynamic pressure generating grooves formed in the bearing surface, if forcible correction is made by using the correction pin, this will result in the correction pin cutting into the hydrodynamic pressure generating grooves in the bearing surface to collapse said grooves, making it impossible to obtain a stabilized hydrodynamic effect. On the other hand, the operation of press-fitting without using the correction pin will fail to provide the necessary alignment between the bearings. Further, Japanese Patent Kokai Heisei 2-107705 discloses an arrangement in which two bearing surfaces are formed in axially spaced from each other and in which a region between the bearing surfaces has a greater diameter than that of the bearing surfaces. This arrangement, though free from the aforesaid problems in practice, cannot prevent the unstable vibrations, such as whirl, because of the lack of hydrodynamic pressure generating grooves in the bearing surfaces.
As for a method of forming hydrodynamic pressure generating grooves in bearing surfaces, such a method has been a known that comprises the steps of inserting into an inner peripheral surface of a bearing blank a shaft-like jig which holds a plurality of circumferencially equispaced balls harder than the bearing blank, imparting a spiral movement to the balls through the rotation and advance of the jig while pressing the balls against the inner peripheral surface of the blanks, thereby to plastically work a region of hydrodynamic pressure generating grooves method of, which method (Japanese Patent 2541208). In this method, the blank bulges in a region adjacent the hydrodynamic pressure generating grooves during forming, and such bulge has to be removed as by lathing or reaming (Japanese Patent Kokai Heisei 8-232958). For this reason, the number of manufacturing steps increases. Further, a driving mechanism and an advancing mechanism for the jig are required, thus complicating the manufacturing equipment.
An object of the present invention is to secure the appropriate circulation of oil between the interior of the bearing body and the bearing clearance to suppress the deterioration of the oil in the bearing clearance, thereby increasing the bearing life, and improving the effect of formation of lubricating oil film in the bearing clearance, thus increasing the bearing rigidity and minimizing the shaft deflection due to imbalance load or the like.
Another object of the invention is to provide an arrangement which is capable of preventing unstable vibrations such as whirl and eliminating the inconveniences (such as the loss of shape of hydrodynamic pressure generating grooves, and axial misalignment) which are involved in the installing operation.
A further object of the invention is to provide a production method which facilitates the forming of a bearing surface having inclined hydrodynamic pressure generating grooves by using simple equipment and fewer steps and with high accuracy.
To achieve said objects, the invention provides a hydrodynamic type porous oil-impregnated bearing comprising a porous bearing body being formed with bearing surface on an inner peripheral surface thereof, and oil retained in pores of the bearing body by impregnation of lubricating oil or lubricating grease, wherein the bearing surface has a first region in which a plurality of hydrodynamic pressure generating grooves inclined in one direction with respect to the axial direction are circumferentially disposed, a second region which is axially spaced from the first region and in which a plurality of hydrodynamic pressure generating grooves inclined in the other direction with respect to the axial direction are circumferentially disposed, and an annular smooth region positioned between the first and second regions. The bearing surface of the bearing body is opposed to an outer peripheral surface of a shaft to be supported, with a bearing clearance defined therebetween. When a relative rotation occurs between the bearing body and the shaft, the hydrodynamic pressure generating grooves mutually reversely disposed in the first and second regions of the bearing surface cause the oil in the bearing clearance to be drawn to the annular smooth region and collect in the latter, so that the oil film pressure in the smooth region is increased. For this reason, the effect of formation of lubricating oil film is high. Further, since the smooth region has no groove formed therein, the bearing rigidity is high as compared with the construction in which hydrodynamic pressure generating grooves axially continuous. Therefore, the shaft deflection can be minimized. Further, it is possible to avoid the lubricating oil film distribution becoming nonuniform owing to variations in surface openings on the bearing surface. By the term xe2x80x9csurface openingsxe2x80x9d is meant those portions of pores of a porous body which open to an outer surface thereof. In the present invention, the surface openings are present in the entire region of the bearing surface including the region formed with the hydrodynamic pressure generating grooves.
Percentage of area of surface openings in the smooth region of the bearing surface is preferably smaller than that of the first and second regions. By the term xe2x80x9cpercentage of area of surface openingsxe2x80x9d is meant the proportion of the total area of the surface openings in unit area of the outer surface. As a result, since the oil which is brought together in the smooth region by the hydrodynamic pressure generating grooves can hardly escape into the interior of the bearing body through the surface openings on the smooth region, the capacity of the produced lubricating oil film can be increased. Further, since an outer peripheral surface of the shaft is supported in a non-contact manner mainly by the lubricating oil film formed of the oil collected in the annular smooth region, the bearing rigidity is high.
The percentage of area of surface openings is in the range of 5-30%, desirably 5-20%, for the first and second regions and 2-20%, desirably 2-15%, for the smooth region. If the percentage of area of surface openings on the first and second regions is less than 5%, the amount of oil to be fed from the interior of the bearing body to the bearing clearance decreases, resulting in insufficient formation of lubricating oil film. Reversely, if it exceeds 30%, the amount of oil which escapes into the interior of the bearing body becomes excessive, resulting in insufficient formation of lubricating oil films on the smooth region. Further, if the percentage of area of surface openings on the smooth region is less than 2%, the production of the bearing becomes difficult, leading to an increase in costs. Reversely, if it exceeds 20%, the amount of oil which escapes into the interior of the bearing body becomes excessive, resulting in insufficient formation of lubricating oil film.
In order to enhance the effect of formation of lubricating oil film on the smooth region, it is preferable that the hydrodynamic pressure generating grooves in the first region and those in the second-region be symmetric with respect to the axial central region of the bearing surface.
At the start or stoppage of rotation, the outer peripheral surface of the shaft comes into instantaneously contact with the bearing surface of the bearing. At this time, they come into contact with each other in the axial end region of the bearing surface. Therefore, by tapering the axial opposite sides of the bearing surface such that the inner diameter increases toward the bearing ends (see FIG. 7), the area of their contact is increased when the apparatus is started or stopped, so that the non-contact state can be instantaneously established. The first and second regions may be tapered throughout or portions (associated with the bearing ends) of each of the first and second regions may be tapered. In addition, the area of the bearing surface other than the tapered surface is parallel with the axis.
In this case, the ratio of an increment xcex94c in the inner diameter from the smooth region to the end of the bearing to the shaft diameter D is xcex94c/D=1/3000-1/200, more desirably, xcex94c/D=1/3000-1/500. If xcex94c/D is less than 1/3000, the resulting taper is too small to prevent instantaneous contact, and if xcex94c/D is greater than 1/200, the resulting taper is too large to provide a useful hydrodynamic effect.
It is possible to provide an arrangement comprising a porous bearing body being formed with a plurality of axially spaced bearing surfaces on an inner peripheral surface thereof, at least one of the plurality of bearing surfaces having inclined hydrodynamic pressure generating grooves, the inner diameter of the region between the bearing surfaces being greater than that of the bearing surfaces, and oil retained in the pores of the bearing body by impregnation of lubricating oil or lubricating grease. Such formation of a plurality of bearing surfaces in a single bearing solves the problem of axial alignment inherent in the case where a plurality of bearings are incorporated as in the prior art. More particularly, since the plurality of bearing surfaces are formed in a single bearing, there is no need to use a correcting pin to obtain axial alignment as in the case of prior art, and the loss of shape of the hydrodynamic pressure generating grooves due the use of such correcting pin does not occur, of course. The formation of inclined hydrodynamic pressure generating grooves in at least one bearing surface effectively prevents unstable vibrations such as whirl.
Provision of a level difference in the boundary between the bearing surface and the region between the bearing surfaces makes it possible to effectively reduce the torque loss in the region between the bearing surfaces.
If the axial section of the region between the bearing surfaces is drawn with a curve which continuous to the bearing surfaces, oil which exudes from the surface openings on the region between the bearing surfaces flows axially along such region, making it easier to feed the oil to the bearing surface, a fact which means effective use of oil and enhancement of formation of lubricating oil film.
The axial section of the region between the bearing surfaces may be drawn with an arc which is greatest in the middle of the region. The oil which has exuded from the surface openings on the region can be easily fed to the bearing surfaces on the opposite sides.
The outer diameter of an outer portion of the bearing body corresponding to at least one bearing surface is determined to be smaller than the outer diameter of an outer portion of the bearing body corresponding to the region between the bearing surfaces, whereby when the bearing body is press-fitted in a housing, deformation of the bearing surfaces under the press-fitting pressure can be prevented or reduced.
The bearing surface having inclined hydrodynamic pressure generating grooves can be formed by the following method: the method comprises the steps of inserting a forming pattern in an inner peripheral surface of a cylindrical porous blank, the forming pattern having a first forming portion for forming a region of hydrodynamic pressure generating grooves and a second forming portion for forming the other regions in the bearing, applying a compacting pressure to the porous blank to press the inner peripheral surface of the porous blank against the forming pattern, thereby simultaneously forming the region of hydrodynamic pressure generating grooves and the other region in the bearing surface on the inner peripheral surface of the porous blank. Alternatively, disposing the forming pattern in a die, filling powder metal material between the forming pattern and the die, applying a compacting pressure to the powder metal material to form a cylindrical compacted body, while simultaneously forming the region of hydrodynamic pressure generating grooves and the other region in the bearing surface on the inner peripheral surface of the compacted body. Release of the forming pattern can be effected by utilizing the spring-back of the porous blank due to removal of the compacting pressure, or by utilizing the spring-back of the compacted body due to removal of the compacting pressure.